Thanks very much for your reply.
I think a variable flowrate will overcomplicate the application. So I will start with a set flowrate and maybe tackle that problem a bit later. So for now, lets assume constant flow rate.
Interesting application you have there! The thermal lag that you have should not be such a big problem for me. And if, I need to fix it. If I need 5-7 minutes to heat up my flow through heater, than the whole application does not make any sense.
To answer you question regarding the deviation, I don't know what is going to be achievable, but I am aiming for as small as possible. And would generally allow larger deviations with rising temperatures. When it comes to temperatures below 40degC. it has to be within the +-1degC bracket.
The heater has two power circuits (400W and 800W) that combined and spread across 5 phases will provide up to 1200W. So switching them on / off at the zero crossings, is how the intended use case looks like. They also mention a constant flow-rate, so we will keep it that way.
Is that how you control your system as well?